The present invention relates to a method for producing a semiconductor device, particularly to a method for producing a semiconductor device in which an impurity concentration profile of an extension region positioned between a channel forming region and each source/drain region can be controlled accurately.
With increasing shrinkage of semiconductor devices, the integration degree is doubled every three years according to a scaling rule, and speed of semiconductor devices is increasing and power consumption thereof is decreasing. The production of finer MOS type FETs has been being accomplished by decreasing a dimension of a gate electrode, decreasing a thickness of a gate insulating layer and highly accurately controlling an impurity concentration profile in a channel forming region or in its vicinity. And, driving capability of semiconductor devices is improved and a parasitic capacitance thereof is decreased according to finer semiconductor devices. In general, in circuits having a CMOS structure, an operating rate is determined depending upon a rate of charging (or discharging) for giving an output of a logic gate at a certain stage to drive a capacitive load in a subsequent logic gate. Therefore, the operating rate is in proportion to the inverse number of capacity of the capacitive load and to the driving capability.
For accomplishing the formation of finer semiconductor devices, conventionally, there has been employed an LDD (lightly doped drain) structure for forming a source/drain region, i.e., a structure having a low-concentration-impurity-containing region which is formed between each source/drain region and a channel forming region and extends from each source/drain region. In this structure, an electric field near a drain region can be reduced and the change of the semiconductor device caused by hot carriers with elapse of time (an increase in threshold voltage Vth and a decrease in driving capability) can be suppressed. In semiconductor devices finely structured in recent years, however, not only it is required to decrease a supply voltage, but also it is rather required to highly accurately control an impurity concentration profile in an impurity-containing region formed between each source/drain region and a channel forming region than it is required to attain reliability with regard to the hot carriers. For decreasing a parasitic capacitance of the impurity-containing region formed between each source/drain region and the channel-forming region, the impurity-containing region is arranged to have relatively high impurity concentration in many cases in recent years. The impurity-containing region formed between each source/drain region and the channel-forming region will be referred to as xe2x80x9cextension regionxe2x80x9d hereinafter in the present specification. The impurity concentration in the extension region is lower than the impurity concentration in the source/drain region in some cases, equal to the same in some cases, and higher than the same in some cases. That is, the impurity concentration in the extension region is determined depending upon the characteristic required of semiconductor devices.
The method of forming a conventional extension region will be explained with reference to schematic partial cross-sectional views of a semiconductor substrate, etc., shown in FIGS. 9A, 9B and 10 hereinafter.
[Step-10]
A device isolation region 11 having a LOCOS structure is formed, for example, in a semiconductor substrate 10 by a known method, followed by well ion-implanting, channel stop ion-implanting and threshold value adjusting ion-implanting. The device isolation region 11 may have a trench structure, or it may be a combination of a LOCOS structure and a trench structure. Then, the surface of the semiconductor substrate 10 is thermally oxidized to form a gate insulating layer 20. Then, a polysilicon layer 21A and a tungsten silicide layer 21B are formed on the entire surface, and the tungsten silicide layer 21B and the polysilicon layer 21A are patterned by a lighography method and a dry etching method, whereby a gate electrode 21 having a polycide structure can be formed on the gate insulating layer 20.
[Step-20]
For forming an extension region 125, then, the exposed silicon semiconductor substrate 10 is ion-implanted (see FIG. 9A). Then, a first heat treatment for activation is carried out for activating the impurity brought by the ion-implantation and recovering the semiconductor substrate 10 from a crystal defect caused by the ion-implantation. When the above first heat treatment for activation is omitted, the impurity brought by the ion-implantation may be abnormally diffused (enhanced diffusion) at a process temperature around 700xc2x0 C. applied during formation of a thin layer, for example, by a chemical vapor deposition method (CVD method) in a subsequent step, due to the crystal defect caused in the silicon semiconductor substrate 10 by the ion-implantation, and the impurity concentration profile may vary to a great extent. In the above manner, the extension region 125 can be formed.
[Step-30]
Then an insulating material layer composed, for example, of SiO2 is deposited on the entire surface by a CVD method, and the insulating material layer is etched back, to form gate-side-walls 122 on the side walls of the gate electrode 21 (see FIG. 9B).
[Step-40]
[Step-40]
For forming source/drain regions, then, the exposed silicon semiconductor substrate 10 is ion-implanted (see FIG. 10), and a second heat treatment for activation is carried out for activating the impurity brought by the ion-implantation, whereby source/drain regions 23 and a channel-forming region 24 interposed between the source/drain regions 23 can be formed. The channel-forming region 24 is located immediately below the gate electrode 21. Each extension region 125 is positioned between each source/drain region 23 and the channel-forming region 24 and extends from each source/drain region 23.
In the method of forming the above conventional extension region, the thermal budget (effective heat treatment quantity) in the extension region is always higher than the thermal budget in the source/drain region for the following reason. The sourcejdrain region is subjected to the heat treatment for activation once, but the extension region is subjected to the heat treatment for activation twice.
For improving the impurity concentration profile in the extension region to make it higher than the impurity concentration profile in the source/drain region, it is required to decrease the impurity concentration in the extension region. When the impurity concentration in the extension region is decreased, however, there is caused the following problem. The resistance of the extension region increases, a parasitic resistance in the semiconductor device increases, and as a result, the driving capability decreases. The parasitic resistance in the extension region and the control of the impurity concentration profile in the extension region have a trade-off relationship, which comes to represent a great difficulty. Further, if the impurity concentration in the extension region is not increased, the driving capability decreases due to an increase in the parasitic resistance. If the impurity concentration is increased to excess, it is difficult to produce a finer-structured semiconductor device with suppressing a short channel effect.
The broadening of the impurity concentration profile in the extension region in a lateral direction particularly increases an overlap capacitance between the marginal portion of the gate electrode and the source/drain region and decreases the operating rate of the semiconductor device to a great extent.
Under the circumstances, the optimization of the impurity concentration and the accurate controlling of the impurity concentration profile in the extension region are increasingly acquiring importance in the production of finer-structured semiconductor devices.
It is therefore an object of the present invention to provide a method for producing a semiconductor device, in which impurity concentration in an extension region positioned between a channel-forming region and each source/drain region can be optimized and an impurity concentration profile in the extension region can be controlled highly accurately.
According to the present invention, the above object of the present invention is achieved by a method for producing a semiconductor device having:
(a) a gate insulating layer formed on a surface of a semi-conductive layer, and a gate electrode formed on the gate insulating layer,
(b) a channel-forming region formed in the semi-conductive layer immediately below the gate electrode,
(c) source/drain regions formed in the semi-conductive layer, so as to sandwich the channel-forming region, and
(d) extension regions, each of which is formed in a region of the semi-conductive layer positioned between each source/drain region and the channel-forming region and extends from each source/drain region,
said method comprising the steps of:
(A) forming the gate insulating layer on the surface of the semi-conductive layer, and then, forming the gate electrode on the gate insulating layer,
(B) introducing an impurity in regions of the semi-conductive layer where the source/drain regions are to be formed, and then, carrying out heat treatment for activation of the introduced impurity, to form the source/drain regions in the semi-conductive layer, and
(C) introducing an impurity into at least regions of the semi-conductive layer where the extension regions are to be formed, and then, carrying out heat treatment for activation of the introduced impurity, to form the extension regions in the semi-conductive layer.
In a conventional method for producing a semiconductor device, the source/drain regions are formed after the extension regions are formed. Therefore, the extension regions are twice heat-treated for activation. In the method for producing a semiconductor device, provided by the present invention, the extension regions are formed after the source/drain regions are formed. Therefore, the number of times of the heat treatment for activation to which each extension region is subjected is smaller than the number of times of the heat treatment for activation to which each source/drain region is subjected. As a result, the impurity concentration profile in the extension region can be highly accurately maintained.
The impurity concentration in the extension region is lower than the impurity concentration in the source/drain region in some cases, is equal to the same in some cases, or is higher than the same in some cases. Essentially, the impurity concentration in the extension region can be determined depending upon the characteristic that the semiconductor device is required to have. The junction depth of the extension region (depth from the surface of the semi-conductive layer to the bottom of the extension region) is required to be smaller than the junction depth of the source/drain region (depth from the surface of the semi-conductive layer to the bottom of the source/drain region).
The gate electrode can be formed of at least a polysilicon layer. That is, the gate electrode may be formed of one polysilicon layer, it may have a two-layered structure formed of a polysilicon layer and a silicide layer (polycide layer), or it may have a two-layered structure formed of a polysilicon layer and a metal layer such as a tungsten layer. In these case, the step (C) preferably includes the step of oxidizing side walls of the polysilicon layer constituting the gate electrode. The thickness of the gate insulating layer in the vicinity of the side walls of the gate electrode can be increased by oxidizing the side walls of the polysilicon layer, and as a result, an overlap capacitance between the marginal portion of the gate electrode and the source/drain region can be decreased.
In the step (C) of the method for producing a semiconductor device, provided by the present invention, after the impurity is introduced into at least the regions of the semi-conductive layer where the extension regions are to be formed, the heat treatment for activation of the introduced impurity may be carried out once only, or may be carried out a plurality of times (at least twice). That is, in the former case, the formation of the extension region is carried out once. In the latter case, the formation of the extension region is carried out a plurality of times (at least twice) from a source/drain region side. In a portion of each extension region closer to the channel-forming region, it is required to control the impurity concentration profile more highly accurately. When the formation of the extension region is carried out a plurality of times, the impurity concentration profile in the extension region can be adjusted or controlled to a more desirable and more highly accurate impurity concentration profile.
When the formation of the extension region is carried out once, there may be employed an embodiment of the method for producing a semiconductor device, provided by the present invention, in which;
between the steps (A) and (B), sidewalls are formed on the side walls of the gate electrode,
in the step (B), the impurity is introduced into the regions of the semi-conductive layer where the source/drain regions are to be formed and the heat treatment for activation of the introduced impurity is carried out, and
in the step (C), the sidewalls are removed, the impurity is introduced into the source/drain regions and the regions of the semi-conductive layer where the extension regions are to be formed and the heat treatment for activation of the introduced impurity is carried out.
By forming the sidewalls as described above, the source/drain regions and the extension regions can be formed in a self-aligned manner. The sidewalls can be composed, for example, of SiO2, SiN, a two-layered structure of SiO2/SiN or a polysilicon.
Alternatively, when the formation of the extension region is carried out a plurality of times, there may be employed an embodiment of the method for producing a semiconductor device, provided by the present invention, in which:
between the steps (A) and (B), sidewalls having a multi-layered structure are formed on the side walls of the gate electrode,
in the step (B), the impurity is introduced into the regions of the semi-conductive layer where the source/drain regions are to be formed and the heat treatment for activation of the introduced impurity is carried out, and
in the step (C), of each sidewall having a multi-layered structure, each sidewall unit, constituting each sidewall having a multi-layered structure, positioned on the outer side is removed, the impurity is introduced into the source/drain regions and the regions of the semi-conductive layer where the extension regions are to be formed, then, the heat treatment for activation of the introduced impurity is carried out, and the removal of such a sidewall unit, the introduction of the impurity and the heat treatment for activation are repeated until the sidewall units constituting the sidewall having a multi-layered structure are all removed.
By forming the sidewall having a multi-layered structure, the formation of the source/drain regions and the formation of the extension regions carried out a plurality of times can be carried out in a self-aligned manner. Each sidewall unit constituting the sidewall having a multi-layered structure can be composed, for example, of SiO2, SiN and a polysilicon, and it is preferred to use a combination of these materials that can allow an etching selectivity.
For decreasing the resistance in the source/drain regions, preferably, gate-side-walls are formed on the side walls of the gate electrode so as to cover at least the extension regions after the step (C), and then, the source/drain regions are converted to silicide source/drain regions. In the conversion to the silicide source/drain regions, surface regions of the source/drain regions may be composed of silicide, or the source/drain regions as a whole in the thickness direction may be composed of silicide depending upon the semiconductor device structures. For the conversion into the silicide source/drain regions, for example, a metal layer is formed on the entire surface, and heat treatment is carried out to react atoms constituting the metal layer with atoms (for example, Si) constituting the semi-conductive layer, whereby a silicide layer is formed. Then, an unreacted portion of the metal layer is removed, and heat treatment is carried out again, whereby a stabilized silicide layer can be obtained. The metal for the metal layer includes cobalt (Co), nickel (Ni), platinum (Pt), titanium (Ti), Ta (tantalum), Mo (molybdenum), tungsten (W) and palladium (Pd). The width of the gate-side-wall on the surface of the semi-conductive layer is required to be equal to, or greater than, the width of the extension region. That is, it is required to fully cover the extension region with the gate-side-wall, and the gate-side-wall may extend up to part of the source/drain region.
The semi-conductive layer may be formed of a silicon semiconductor substrate per se, or it may be formed of a so-called SOI (Semiconductor-On-Insulator) layer formed on an insulation layer formed on a support. When the semi-conductive layer is formed of a silicon semiconductor substrate per se, a semiconductor device is a so-called bulk semiconductor device, and when it is formed of an SOI layer, a semiconductor device is a so-called SOI type semiconductor device. The semi-conductive layer may be made of Si, or it may be made of a compound crystal of Sixe2x80x94Ge.
The method of forming the SOI layer includes the following methods (1) to (5).
(1) A substrate bonding method, in which a semiconductor substrate and a supporting substrate are bonded through an insulation layer, and the semiconductor substrate is ground and polished from its back surface, to obtain a support made of the supporting substrate, the insulation layer and the semi-conductive layer made of the ground and polished semiconductor substrate.
(2) A smart-cut method, in which an insulation layer is formed on a semiconductor substrate, the semiconductor substrate is implanted with hydrogen ion to form a peel-off layer inside the semiconductor substrate, the semiconductor substrate and a supporting substrate are bonded through the insulation layer, the resultant product is heat-treated to peel off (cleave) the semiconductor substrate from the peel-off layer, and the remaining semiconductor substrate is ground and polished from its back surface, to obtain a support made of the supporting substrate, the insulation layer and the semi-conductive layer made of the ground and polished semiconductor substrate.
(3) An SIMOX (Separation by IMplanted OXygen) methods in which oxygen ion is implanted into a semiconductor substrate, and then, the semiconductor substrate is heat-treated to form an insulation layer inside the semiconductor substrate, whereby a support made of part of the semiconductor substrate is formed below the insulation layer and a semi-conductive layer made of part of the semiconductor substrate is formed above the insulation layer.
(4) A method in which a single crystal semi-conductive layer is formed on an insulation layer formed on a semiconductor substrate corresponding to a support, in a gaseous phase or a solid phase, to obtain a support formed of the semiconductor substrate, the insulation layer and the semi-conductive layer formed of the single crystal semi-conductive layer.
(5) A method in which an insulation layer is formed by partially converting a surface of a semiconductor substrate into a porous surface by anodic oxidation, to form a support made of part of the semiconductor substrate below an insulation layer and a semi-conductive layer made of part of the semiconductor substrate above the insulation layer.
The method for producing a semiconductor device, provided by the present invention, is effective particularly when it is applied to the production of a semiconductor device in which the junction depth of the source/drain region is determined by a physical parameter such as the thickness of the SOI layer, that is, an SOI type semiconductor device. On the production of the SOI type semiconductor device, when the heat treatment for activation is carried out, a change in the impurity concentration profile in a lateral direction increases as a change in the impurity concentration profile in the thickness direction is small. However, the impurity concentration profile in the lateral direction can be accurately controlled by applying the method for producing a semiconductor device, provided by the present invention.
The gate insulating layer can be formed, for example, by a thermal oxidation method or a combination of a thermal oxidation method with a thermal nitriding method, and it can be composed, for example of SiO2 or SiO2/SiN.